Improve sov_schema_db documentation

[neysofu]

Description

I added doc comments and two doc tests for sov_schema_db, while focusing on documenting the correct usage for seeking, which can be quite bug-prone as we've seen recently.

Linked Issues

• See also Fix ledger rpc getTransactions bug #417.

Testing

#![deny(missing_docs)] and doc tests.

[citizen-stig]

@neysofu , do you know why doctests are ignored in our test CI step:

https://github.com/Sovereign-Labs/sovereign-sdk/actions/runs/5465623913/jobs/9949332002#step:6:685

   Doc-tests sov-db

running 2 tests
test src/schema/tables.rs - schema::tables::define_table_with_default_codec (line 124) ... ignored
test src/schema/tables.rs - schema::tables::define_table_without_codec (line 54) ... ignored

[codecov]

Codecov Report

Merging #463 (55a6611) into main (71dd233) will decrease coverage by 0.1%.
The diff coverage is 95.2%.

Impacted Files	Coverage Δ
full-node/db/sov-db/src/ledger_db/mod.rs	91.2% <ø> (ø)
full-node/db/sov-schema-db/src/iterator.rs	100.0% <ø> (ø)
full-node/db/sov-schema-db/src/schema.rs	0.0% <ø> (ø)
full-node/db/sov-db/src/schema/tables.rs	85.0% <90.0%> (ø)
full-node/db/sov-schema-db/src/lib.rs	90.7% <96.7%> (+0.2%)	⬆️
full-node/db/sov-db/src/lib.rs	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

[neysofu]

@citizen-stig those two doc tests were previously marked as ignore, they're not the ones I added in this PR. I'll fix those too though, good catch

[neysofu]

Nevermind, those doc tests rely on private components so ignore was a good call. Merging.